Impregnation of a lignocellulosic material

ABSTRACT

A composition for impregnation of a lignocellulosic material such as wood or wood particles, comprises a high boiling point tar acid or a pitch derived from coal; a solvent for the tar acid or pitch selected from a low carbon alcohol, an aceotrope of a low carbon alcohol and another solvent, and a mixture of a lower carbon alcohol and water; a phenol formaldehyde resole resin; an acid catalyst for the phenol formaldehyde resole resin; and preferably a formaldehyde donor or a precursor thereof. There is also disclosed a method of preparing a finished product from a lignocellulosic material by using the impregnating composition.

This application is the national phase of international applicationPCT/GB99/00841 filed Mar. 18, 1999 which designated the U.S.

BACKGROUND OF THE INVENTION

This invention relates to a composition for impregnation of alignocellulosic material, and to a method of preparing a finishedproduct from a lignocellulosic material which has been so impregnated.

It is well known to preserve timber by impregnating preservatives in apressure cylinder, usually with a vacuum/pressure, vacuum/vacuum,pressure/pressure or pressure/pressure/vacuum cycle. Examples ofsuitable preservatives are the copper chrome arsenates in water,creosotes, and MDIs with or without anhydrides in non aqueous solvents.

Desirable properties to be imposed upon wood are resistance to wateringress, resistance to the movement of water in the material bycapillarity, preventing attack of the lignocellulosic material by microorganisms, particularly fungi or insects such as termites, stabilisingthe wood dimensionally, improving its mechanical properties and possiblyalso improving its appearance. Very few of the known impregnationtechnologies accomplish all of these objectives.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, there is provided acomposition for impregnation of a lignocellulosic material, whichcomposition comprises:

(a) a high boiling point tar acid or a pitch derived from coal;

(b) a solvent for component (a) selected from the group consisting of alow carbon alcohol, an azeotrope of a low carbon alcohol and anothersolvent, and a mixture of a lower carbon alcohol and water;

(c) a phenol formaldehyde resole resin;

(d) optionally an acid catalyst for the phenol formaldehyde resoleresin; and

(e) optionally a formaldehyde donor or a precursor of formaldehyde.

By a lignocellulosic material there is meant any plant materialemanating from the photosynthetic phenomenon.

The lignocellulosic material may, for example, be chosen from wood insawn, sliced, peeled or veneer form; plywood; paper; woven strands;fibres, particles, flakes or chips of wood or agricultural products; orthe like.

A preferred impregnating composition of the invention comprises:

(a) a high boiling point tar acid or a pitch derived from coal in anamount of from 2% to 60% inclusive by mass, more preferably from 8% to15% inclusive by mass of the combined mass of components (a), (b) and(c);

(b) a solvent selected from the group consisting of a low carbonalcohol, e.g. methanol, ethanol, propanol or butanol, preferablymethanol; an azeotrope of a low carbon alcohol and another solvent, anda mixture of a low carbon alcohol and water; in an amount of from 50% to97% inclusive by mass, more preferably from 77% to 90% inclusive by massof the combined mass of components (a), (b) and (c);

(c) a phenol formaldehyde resole resin in an amount of from 1% to 30%inclusive by mass, more preferably from 2% to 8% inclusive by mass ofthe combined mass of components (a), (b) and (c);

(d) an acid catalyst for the phenol formaldehyde resole resin in anamount of from 1% to 15% inclusive by mass, more preferably from 4% to6% inclusive by mass of the resin; and

(e) a formaldehyde donor or a precursor of formaldehyde, such ashexamethylene tetramine, in an amount of from 1% to 15% inclusive bymass, more preferably from 8% to 12% inclusive by mass of the mass ofthe high boiling point tar acid or the pitch.

The solvent, component (b), also acts as an extending liquid for thephenol formaldehyde resole resin.

The impregnating composition may also contain other components such asan antioxidant and a deodoriser, both of which should be miscible orsoluble in the extending liquid.

A preferred level of impregnation for upgrading wood for exteriorexposure is, for example, 2% by mass of resin with 10% by mass of pitchon the mass of the dry wood.

According to a second aspect of the invention, there is provided amethod of preparing a finished product from a lignocellulosic material,including the steps of:

(i) impregnating the lignocellulosic material with an impregnatingcomposition as set out above;

(ii) removing the solvent from the product of step (i), optionally withrecovery thereof for reuse; and

(iii) subjecting the product of step (ii) to suitable conditions oftemperature and optionally of pressure to polymerise or to set theresin, to form the finished product.

Step (i) of the method of the invention may be carried out by immersingthe lignocellulosic material in the impregnating composition, or byimpregnating the lignocellulosic material with the impregnatingcomposition under suitable conditions of vacuum and pressure.

In step (ii) of the method of the invention, the solvent may be removedfrom the product of step (i) in any suitable way.

In step (iii) of the method of the invention, the product of step (ii)may be subjected simply to an elevated temperature, for example, atemperature exceeding 90° C., to allow the resin to polymerise or set.Alternatively, when the lignocellulosic material is in particulate form,the product of step (ii) may be subjected to suitable conditions oftemperature, e.g. a temperature exceeding 90° C., and pressure, forexample, between the platens of a press, to allow the resin topolymerise or set and thus to adhere the particles to one another toform the finished product.

According to a third aspect of the invention, there is provided afinished product comprising a lignocellulosic material which has beenimpregnated with an impregnating composition as set out above, theimpregnated lignocellulosic material having been subjected to suitableconditions of temperature and optionally of pressure to polymerise or toset the resin, to form the finished product.

DESCRIPTION OF EMBODIMENTS

The crux of the invention is a composition for impregnation of alignocellulosic material which comprises a high boiling point tar acidor a pitch derived from coal, a solvent therefor, and a suitablecompatible binder to bind the tar acid or the pitch in thelignocellulosic material.

The first component of the impregnating composition of the invention isthe high boiling point tar acid or the pitch derived from coal,particularly derived from the water stream in the distillationclassification of coal.

During the distillation of coal, there is produced gas, tars and a waterstream containing high boiling point tar acids and pitches. It is theselatter products which are useful in the impregnating composition of theinvention.

It is important that the high boiling point tar acid or pitch ispartially or completely dissolved in the solvent in order thatimpregnation of the lignocellulosic material occurs.

An example of a suitable tar acid is Merisol XHBTA produced byMerichem-Sasol RSA (Pty) Limited, which product has a boiling point inthe range 210° C. to 300° C. at atmospheric pressure, and a density of1,03. This product is only sparingly soluble in water, but readilysoluble in the low carbon alcohols and acetates. It has a composition of20% to 30% ethyl phenols, 40% to 60% xylenols, and 15% to 25% highboiling point tar acids.

A further suitable material is a phenosolven pitch of a density in therange 1.01 to 1.06, and with a composition of 12% higher boiling pointtar acids, 12% N cresol. 21.8% M & P cresol, 8% xylenols and the balancebeing phenols, and which is substantially soluble in methanol.

The function of this component is to impose resistance to water ingresson the lignocellulosic material and to prevent attack of thelignocellulosic material by microorganisms, particularly fungi, orinsects, or the like.

The second component of the impregnating composition of the invention isa solvent for the tar acid or the pitch. The solvent may be a low carbonalcohol such as methanol, ethanol, propanol or butanol, preferablymethanol; an azeotrope of a low carbon alcohol, preferably methanol andanother solvent, i.e. methyl acetate 80%/methanol 19% with a boilingpoint of 54° C. or an azeotrope with ethylacetate or acetone; or amixture of a low carbon alcohol, preferably methanol, and water.

The solvent also acts as an extending liquid for the phenol formaldehyderesole resin.

Thus the function of the solvent or extending liquid is also to preventresin polymerisation in the impregnating composition and in theimpregnated lignocellulosic material until the solvent or extendingliquid has been removed.

In addition, the solvent or extending liquid serves to control theviscosity of the impregnating composition. The solvent or extendingliquid also allows control of the impregnation composition solids thatare left in the impregnated material, which is important commerciallyand which allows control of the final properties of the finishedproduct.

The third component of the impregnating composition is a phenolformaldehyde resole resin. The phenol formaldehyde resole resin may beone that is a water borne and water dilutable thermosetting phenolformaldehyde resin, examples being Schenectady International SG 3100,HRJ-4173 with solids percentages between 65% and 82% and Code J 2018 Lby Blagden-Cellubond Limited of the UK. The resin may be produced athigh solids content in a water miscible solvent, making the resin waterdilutable and allowing water compatibility of the less polar or highmolecular weight phenolic resins. A true solution is preferred to adispersion or emulsion in order to achieve true penetration and fibrewetting of the lignocellulosic material and avoiding surface blockingand accumulation on impregnation.

The function of the phenol formaldehyde resole resin is to contribute tothe hardness and dimensional stability of the finished product, toimprove the mechanical properties thereof, and to bind the high boilingpoint tar acid or the pitch into the matrix or cellular structure of thelignocellulosic material.

The fourth optional component of the impregnating composition of theinvention is an acid catalyst for the phenol formaldehyde resole resin.Examples of suitable catalysts are Phencat 10 and Phencat 15 byBlagden-Cellubond Limited, and catalysts based on sulphonic acid.

The function of the acid catalyst is to promote the polymerisation ofthe phenol formaldehyde resole resin once the solvent or extendingliquid has been removed.

The resin may however polymerise simply with the application of heat.

The fifth optional component of the impregnating composition of theinvention is a formaldehyde donor or a precursor of formaldehyde, suchas for example hexamethylene tetramine, methylene glycol, trimethyleneglycol and paraformaldehyde.

Thus, for example, hexamethylene tetramine serves as a formaldehydedonor to facilitate the polymerisation of the phenol formaldehyde resoleresin and reactive elements in the high boiling point tar acids andpitches, and promote the consolidation of these compounds to thechemical makeup of the phenol formaldehyde resole resin.

The second aspect of the invention is a method of preparing a finishedproduct from a lignocellulosic material.

The first step of the method is to impregnate the lignocellulosicmaterial with an impregnating composition as set out above.

The impregnation may take place by immersion or by impregnation underpressure. Various examples of methods of impregnation are set out below:

1A Placing the lignocellulosic material in a pressure cylinder;subjecting it to a vacuum, generally to the lowest possible pressureconsistent with its moisture percentage and for periods of time,typically of 5 to 25 minutes; flooding the cylinder with theimpregnating composition; subjecting the cylinder to a positive pressurein the range of 1 to 25 bar, but more generally in the range of 2 to 8bar, for a period of time such as 10 to 40 minutes; removing theimpregnating composition from the cylinder and withdrawing the material.

1B Placing the lignocellulosic material in a pressure cylinder;subjecting it to a vacuum, the level of which is governed by themoisture percentage in the lignocellulosic material, the length of timeunder vacuum being from 5 to 30 minutes; flooding the cylinder with theimpregnating composition; allowing a dwell time for penetration for upto 40 minutes and then removing the impregnating composition.

1C. Where limited penetration of the impregnating composition into thelignocellulosic material is required, placing the material in a pressurecylinder and subjecting it to a positive gas pressure, the pressurebeing between 0,5 and 3 bar, for 5 to 20 minutes; flooding the cylinderwhile still under pressure with the impregnating composition;maintaining the pressure in the cylinder during this operation at aconstant level; once full, subjecting the contents of the cylinder to apositive pressure exceeding the first pressure and being in the range offrom 1,5 bar to 8 bar, the differential between the first and secondpressures controlling the depth of penetration into the lignocellulosicmaterial; removing the impregnating composition and then subjecting thelignocellulosic material to a vacuum for period of from 5 to 20 minutesin order to release the internal air pressure in the lignocellulosicmaterial, thus minimising subsequent kick back or exudation of thecomposition from the impregnated material.

1D Subjecting the lignocellulosic material to any of the processes 1A to1C above, and then repeating the process particularly having subjectedthe impregnated material between treatments to a vacuum in order toremove as much of the solvent between cycles as is possible. This stepincreases the level of chemical modifiers left in the lignocellulosicmaterial.

1E Where the lignocellulosic material is in thin sheet form such aspaper in weights of from 100 to 600 g/m², being either virgin kraft orrecycled paper chip or kraft, or woven hessian and the like or in chipor particle form, or thin veneer, immersing the material in theimpregnating composition.

1F Where the lignocellulosic material is in particle form, placing it ina mixer; adding the impregnating composition on a pre-determined weightbasis; mixing rapidly, generally not exceeding 3 minutes in time; andremoving the impregnated particles from the mixer.

The second step of the method of the invention is to remove the solventfrom the product of step (i), optionally with recovery thereof forreuse.

The recovery of the solvent from the treated material may be carried outby placing it in a suitable kiln or drier and subjecting it to gasmovement, preferably air, at a temperature propagating the most rapidand economical evaporation of the solvent from the treated material,until this process is complete, followed by escalating temperature toensure the completion of the chemical reactions required.

The third step of the method of the invention is to subject the productof step (ii) to suitable conditions of temperature and pressure topolymerise the phenol formaldehyde resole resin, to form the finishedproduct.

For example, when the lignocellulosic material is in sheet form, thenthe material may be subjected to conditions of elevated temperature,preferably at a level exceeding 90° C., to induce polymerisation of theresin.

Alternatively, when the lignocellulosic material is in particulate form,the particles may be subjected to a combination of elevated temperature,preferably exceeding 90° C., and pressure, for example, between theplatens of a press, to induce the resin to polymerise and to form thefinished product. In this way, chipboard, oriented strand board, mediumdensity fibre board, flake board and the like can be manufactured.

Examples of the invention will now be given.

Example 1

100 kg of phenol formaldehyde resole resin Code J 2018 L by Poly ResinProducts South Africa is added to 600 kg of methanol. 60 kg each ofphenosolven pitch from Sasol Tar Division of Sasol Chemicals, SouthAfrica, and 60 kg of XHBTA Higher boiling point tar acid byMerichem-Sasol RSA (Pty) Ltd, are blended. There is added to this blend8 kg of hexamethylene tetramine and this mix is added to and dissolvedin the resin/methanol composition. Finally, 9 kg of acid catalystPhencat 10 by PRP Resins South Africa is added. Lengths of pine being 1m long, 22 mm thick and 150 mm wide, are placed in a pressure cylinderand subjected to a vacuum for 15 minutes. A final pressure of 15 kPa isachieved. The impregnating composition is now introduced into thecylinder and subjected to a pressure of 6 bar for 25 minutes. Theaverage uptake of the impregnating composition per dry kg of pine is inthe range of 875 g to 1.1 kg. The charge is removed from theimpregnating cylinder and placed in an extractor kiln where the methanolis removed, using a circulating closed loop with air velocity of 5meters per second at a temperature of 40° C. Once the methanol has beenremoved the temperature is raised progressively to 110° C. until thechemical reactions in the material have gone to completion. Theresulting pine has improved resistance to water penetration and movementin the material by capillarity. It is disinfected to the point of beingproof to either fungi or termites and the hardness of the material hasbeen increased from typically 2500 Newtons on the Janka Ball Test to 5500 Newtons. The resulting wood is fit for exterior ground contactapplications.

Example 2

A mixture comprising of one part of J 2018 L phenol formaldehyde resoleresin, four parts of a 50% blend between XHBTA high boiling point taracid and phenosolven pitch from Sasol, 0,4 parts hexamethylene tetramineand 0,75 parts of methanol are blended, and then 0,08 parts of acidcatalyst Phencat 10 are added in an in-line mixer and sprayed onto woodparticles at the rate of 10% on a mass basis. To the damp wood particlesis now added 3% on a mass basis of a phenol formaldehyde novolac resinin finely divided dry powder form V12 Code Cellubond CH 113 byBlagden-Cellubond Ltd. of the UK. The wood chips progress continuouslythrough a drier, removing all of the methanol which is scrubbed from theemitted air. The wood particles are now passed through a forming headwhich lays down a mat on a moving belt at the rate by mass of 11,5kg/m². This mat is then pressed between heated press platens at apressure of 25 kg/cm² making a particle board of a density of 720 kg/m³and a thickness of 16 mm. The properties of the board are a muchincreased resistance to water swell on submersion. The composite isproof to attack by termites or fungi and has a much improved internalbond strength.

In the two examples above, the Rideal-Walker coefficient is anticipatedto be between 5 and 16. The standard Rideal-Walker (RW) test comparesthe amount of dilution possible with the test sample to obtain the samebacterial or fungicidal efficiency as a 1 gram in 100 ml phenol sample.The ratio of the dilution gives the RW coefficient. For an example, if adilution of 1:1500 of the test solution gives the same “kill” as 1:100solution of phenol, the test sample has an RW of 15.

Phenolics have bactericidal and fungicidal properties and their strengthtends to increase as their chain length increases. For example phenolhas an RW coefficient of 1, M-Cresol has an RW coefficient of 2.3 and2,6-xylenol has an RW coefficient of 3.8. Merisol XHBTA contains theheavier phenolics and has been tested to have RW coefficients of between14.1 and 15.7. XHBTA contains approximately 15% 2,4/2,5-xylenol, 55% ofthe heavier xylenols and ethyl phenols, 25% high boiling point tar acidand 5% of pitch. It also acts as a hydrophobic agent and other coal tarderivatives with a higher pitch percentage can usefully be blended withthis material.

What is claimed is:
 1. A composition for impregnation of alignocellulosic material, the composition comprising: (a) either (i) ahigh boiling point tar acid derived from a water stream produced duringthe distillation classification of coal; or (ii) a pitch derived from awater stream produced during the distillation classification of coal;(b) a solvent for component (a) selected from the group consisting of alow carbon alcohol selected from the group consisting of methanol,ethanol, propanol and butanol, an azeotrope of the low carbon alcoholand another solvent, and a mixture of the low carbon alcohol and water;(c) a resin component consisting essentially of phenol formaldehyderesole resin; (d) optionally an acid catalyst for the phenolformaldehyde resole resin; and (e) optionally a formaldehyde donor or aprecursor of formaldehyde.
 2. A composition according to claim 1comprising: (a) the high boiling point tar acid or the pitch in anamount of from 2% to 60% inclusive by mass of the combined mass ofcomponents (a), (b) and (c); (b) the solvent is an amount of from 50% to97% inclusive by mass of the combined mass of components (a), (b) and(c); (c) the phenol formaldehyde resole resin in an amount of from 1% to30% inclusive by mass of the combined mass of components (a), (b) and(c); (d) the acid catalyst in an amount of from 1% to 15% by mass of theresin; and (e) the formaldehyde donor or the precursor of formaldehydein an amount of from 1% to 15% inclusive by mass of the mass of the highboiling point tar acid or the pitch.
 3. A composition according to claim2 comprising the high boiling point tar acid or the pitch in an amountof from 8% to 15% inclusive by mass of the combined mass of components(a), (b) and (c).
 4. A composition according to claim 2 or claim 3comprising the solvent in an amount of from 77% to 90% inclusive by massof the combined mass of components (a), (b) and (c).
 5. A compositionaccording to claim 2 comprising the phenol formaldehyde resole resin inan amount of from 2% to 8% inclusive by mass of the combined mass ofcomponents (a), (b) and (c).
 6. A composition according to claim 2comprising the acid catalyst in an amount of from 4% to 6% inclusive bymass of the resin.
 7. A composition according to claim 2 comprising theformaldehyde donor or the precursor of formaldehyde in an amount of from8% to 12% inclusive by mass of the mass of the high boiling point taracid or the pitch.
 8. A composition according to claim 1 wherein thesolvent is selected from a group consisting of methanol, an aceotrope ofmethanol and another solvent, and a mixture of methanol and water.
 9. Acomposition according to claim 1 wherein the formaldehyde donor or theprecursor of formaldehyde is selected from the group consisting ofhexamethylene tetramine, methylene glycol, trimethylene glycol, andparaformaldehyde.
 10. A method of preparing a finished product from alignocellulosic material, including the steps of: (1) impregnating thelignocellulosic material with an impregnating composition comprising:(a) either (i) a high boiling point tar acid derived from a water streamproduced during the distillation classification of coal; or (ii) a pitchderived from a water stream produced during the distillationclassification of coal; (b) said component (a) being partially orcompletely dissolved in a solvent therefor selected from the groupconsisting of a low carbon alcohol selected from the group consisting ofmethanol, ethanol, propanol and butanol, an azeotrope of the low carbonalcohol and another solvent, and a mixture of the low carbon alcohol andwater; (c) a resin component consisting essentially of a phenolformaldehyde resole resin; (d) optionally an acid catalyst for thephenol formaldehyde resole resin; and (e) optionally a formaldehydedonor or a precursor of formaldehyde; (2) removing the solvent from theproduct of step (1), optionally with recovery thereof for re-use; and(3) subjecting the product of step (2) to suitable conditions oftemperature and optionally of pressure to polymerise or to set theresin, to form the finished product.
 11. A method according to claim 10wherein step (1) is carried out by immersing the lignocellulosicmaterial in the impregnating composition or by impregnating thelignocellulosic material with the impregnating composition undersuitable conditions of vacuum and pressure.
 12. A method according toclaim 10 or claim 11 wherein in step (3) the product of step (2) issubjected to an elevated temperature to allow the resin to polymerise orto set.
 13. A method according to claim 10 or claim 11 wherein in step(3) the product of step (2) is subjected to an elevated temperature andpressure to allow the resin to polymerise or to set.
 14. A methodaccording to claim 10 wherein the impregnating composition comprises:(a) the high boiling point tar acid or the pitch in an amount of from 2%to 60% inclusive by mass of the combined mass of components (a), (b) or(c); (b) the solvent in an amount of from 50% to 97% inclusive by massof the combined mass of components (a), (b) and (c); (c) the phenolformaldehyde resole resin in an amount of from 1% to 30% inclusive bymass of the combined mass of components (a), (b) and (c); (d) the acidcatalyst in an amount of from 1% to 15% by mass of the resin; and (e)the formaldehyde donor of the precursor of formaldehyde in an amount offrom 1% to 15% inclusive by mass of the mass of the high boiling pointtar acid or the pitch.
 15. A method according to claim 14 wherein theimpregnating composition comprises the high boiling point tar acid orthe pitch in an amount of from 8% to 15% inclusive by mass of thecombined mass of components (a), (b) and (c).
 16. A method according toclaim 14 wherein the impregnating composition comprises the solvent inan amount of from 77% to 90% inclusive by mass of the combined mass ofcomponents (a), (b) and (c).
 17. A method according to claim 14 whereinthe impregnating composition comprises the phenol formaldehyde resoleresin in an amount of from 2% to 8% inclusive by mass of the combinedmass of components (a), (b) and (c).
 18. A method according to claim 14wherein the impregnating composition comprises the acid catalyst in anamount of from 4% to 6% inclusive by mass of the resin.
 19. A methodaccording to claim 14 wherein the impregnating composition comprises theformaldehyde donor or the precursor of formaldehyde in an amount of from8% to 12% inclusive by mass of the mass of the high boiling point taracid or the pitch.
 20. A method according to claim 10 wherein thesolvent of the impregnating composition is selected from the groupconsisting of methanol, an azeotrope of methanol and another solvent,and a mixture of methanol and water.
 21. A method according to claim 10wherein the formaldehyde donor or the precursor of formaldehyde isselected from the group consisting of hexamethylene tetramine, methyleneglycol, trimethylene glycol, and paraformaldehyde.
 22. A finishedproduct comprising a lignocellulosic material which has been impregnatedwith an impregnating composition comprising: (a) either (i) a highboiling point tar acid derived from a water stream produced during thedistillation classification of coal; or (ii) a pitch derived from awater stream produced during the distillation classification of coal;(b) said component (a) being partially or completely dissolved in asolvent therefor selected from the group consisting of a low carbonalcohol selected from the group consisting of methanol, ethanol,propanol and butanol, an azeotrope of the low carbon alcohol and anothersolvent, and a mixture of the low carbon alcohol and water; (c) a resincomponent consisting essentially of a phenol formaldehyde resole resin;(d) optionally an acid catalyst for the phenol formaldehyde resoleresin; and (e) optionally a formaldehyde donor or a precursor offormaldehyde, the impregnated lignocellulosic material having beensubjected to suitable conditions of temperature and optionally ofpressure to polymerise or to set the resin, to form the finishedproduct.